The nephritic factor of the alternative pathway of complement (NeFa) has been found in the sera of patients with membranoproliferative glomerulonephritis (MPGN) and partial lipodystrophy (PLD) and has been described as a factor which is able to induce cleavage of the third component of complement (C3) in normal human serum through the alternative pathway. It has been demonstrated that NeFa binds to and stabilizes C3bBb (alternative C3 convertase). NeFa appears to be antigenically and structurally similar to IgG and therefore it might be an autoantibody directed against C3bBb complex. Sera from patients with systemic lupus erythematosus (SLE) contain autoantibodies which bind and stabilize the C3 convertase of the classical pathway. This is classical pathway nephritic factor (NeFc). The relation between the development of renal lesions and the NeFa mediated persistent hypocomplementemia remains unexplained. To study the production of nephritic factors, we isolated B lymphocytes from peripheral blood mononuclear cells from patients with MPGN, SLE and normal individuals and established B cell lines by infecting them with Epstein-Barr virus (EBV) containing supernatants. We found that EBV transformed B cell lines derived from patients with MPGN, but not from normal individuals, produce an IgG molecule which stabilizes that C3bBb convertase activity. Supernatants from EBV transformed B cell lines from patients with SLE contain IgG molecules which stabilize C4b2a convertase activity. Full chemical and functional characterization of these antibodies to convertases is in progress.